1. Field of Invention
The invention relates generally to electronics rework, and more particularly to a solder tool for rework of high-density circuits and methods of using the same.
2. Related Art
To meet the demand for advanced electronic devices such as wireless handsets and personal digital assistants (PDAs), manufacturers are continually increasing the density of packaged integrated circuits. For example, chip scale package technologies provide a small footprint on the printed circuit board (PCB), allowing manufacturers to pack more electronics into a given space. One type of chip scale package, known as the Bump Chip Carrier(trademark) (BCC) has become quite popular for use in wireless handsets not only because of its small footprint but because of superior RF properties. The BCC design does not use traditional pins or leads to connect the chip to its PCB. Instead, a BCC chip has conducting pads (the xe2x80x9cbumpsxe2x80x9d) that attach to solder on the PCB. A similar leadless design is found in land grid array chip packages. Other leadless packaging devices are known and used in the electronics field.
Before a board is populated with such leadless chips, a stencil is typically used to apply solder to pads on the board that correspond with I/O pads on the chip. When IC chips are placed on the board, the solder paste assists in adhering the chips to the board. Upon heating, the chips"" pads bond to the solder on the board.
The resulting populated boards may be relatively expensive. For example, a populated board designed for use in a wireless handset often may exceed $50 in cost to manufacture. Unfortunately, some of the populated boards do not function properly. Quite often, such failures result from just a single IC chip malfunctioning or being misplaced on the board. Discarding a populated board merely because a single IC chip on the board is non-functional would thus be wasteful. Rather than discarding boards having a defective IC chip, an attempt is made to replace the defective chip in a process known as rework.
The rework process, which has always been labor intensive, is made even more difficult when using high-density leadless chips. For example, once a defective IC chip is removed from the board, the original stencil used for soldering the board cannot be reused because of the other functional chips still attached to the board. Instead, a rework technician must place a smaller stencil onto the space originally occupied by the defective chip. The technician must place the stencil, carefully apply solder paste, position a new IC chip, and heat the repopulated board to activate the solder. Because modern boards typically have very fine pitch (the spacing between traces) to save space, the proper positioning of this smaller stencil is very difficult to achieve. Further, it is difficult for the technician to accurately control the amount and placement of the solder paste. For example, adding just a little too much solder paste, or positioning the stencil slightly off target may result in a short circuit between pads on the chip. Such a short or other malfunction causes the rework attempt to fail and the entire board and the new IC chip are thereby generally discarded. In this fashion, the value of the board and chip is lost, the rework labor wasted. Because of these rework difficulties, even a skilled technician rarely achieves better than a 10% rework success rate for fine-pitch BCC-populated boards.
Accordingly, there is a need in the art for improved rework tools and techniques for boards populated by leadless integrated circuits.
It is desirable to rework a circuit board having a defective leadless chip without requiring the use of a stencil to apply solder paste to the board. It is therefore an object of the invention to provide a tool that can be used to enable efficient rework using high-density electronic packages. The resulting soldered chips may be placed and secured on the board.
To overcome the disadvantage in the prior art and meet the objectives of this invention, a solder paste tool and method for using the same is herein disclosed and claimed.
The tool comprises a plate having recesses adapted to receive leadless chips. Within each recess, apertures are arranged according to pads on the leadless chips. To use the tool, the leadless chips are placed in the recesses. Solder paste may then be applied through the apertures to a portion of the chip pads. Upon heating, the applied solder paste forms solder balls on the pads of the leadless chips.
Advantageously, the rework solder tool enables efficient rework of high density chip packages. The rework solder tool may be used to obtain a more precise and controlled arrangement of solder on a high-density chip. In such a manner, the pre-soldered chip may be more efficiently and accurately positioned on a printed circuit board. Accordingly, the rework solder tool provides the desirable result of having fewer discarded boards, and a higher return of rework resources as compared to known rework systems.